This paper describes a novel method for improving the navigation and guidance of devices and catheters in electrophysiology and interventional cardiology procedures using volumetric data fusion. The clinical workflow includes the acquisition and reconstruction of CT data from a C-arm X-ray angiographic system and the real-time acquisition of volumetric ultrasound datasets with a new intracardiac real-time 3D ultrasound catheter. Mono- and multi-modal volumetric registration methods, as well as visualization modes, that are suitable for real-time fusion are described, which are the key components of this work. Evaluation on phantom and in-vivo animal data shows that it is feasible to register and track the motion of real-time 3D intracardiac ultrasound in C-arm CT.